1. Field of the Invention
The present invention relates to an apparatus and method for controlling black stretch in a video signal processing system, and more particularly, to an apparatus and method for adaptively varying a black stretch control range and a gain according to a mean level of a video signal to control black stretch of the video signal. The present application is based on Korean Patent Application No. 2001-63251, filed Oct. 13, 2001, which is incorporated herein by reference.
2. Description of the Related Art
Generally, a dark video signal is more sensitive than a bright video signal in a video signal processing system, and a screen looks like it is floating when there is no video signal in a black area. To overcome this phenomenon, the black level of a dark video signal is stretched using a black stretch control apparatus to enhance contrast.
As shown in FIG. 1, a conventional black stretch control apparatus includes a minimum detector 110, a gain calculator 120, a comparator 130, a black stretch calculator 140, and a multiplexer 150.
The minimum detector 110 detects a minimum value of an input video signal Yi in units of frames. The minimum may be obtained by performing horizontal and vertical filtering on the input video signal Yi or sequentially comparing luminance levels of individual pixels.
Then, the gain calculator 120 calculates a slope of black stretch using the detected minimum value and a′ tilt value TILT corresponding to a maximum of a predetermined black stretch control range. In other words, as shown in FIG. 2, the slope of a line S0′ is calculated on the basis of the minimum value. In FIG. 2, a line S0 shows input/output characteristics when gain is 1 before black stretch is performed.
Next, the black stretch calculator 140 calculates a black-stretched signal by multiplying the input video signal Yi by the slope calculated by the gain calculator 120.
The comparator 130 compares the input video signal Yi with the tilt value TILT and outputs a “high” level signal if the input video signal Yi is less than the tilt value TILT. If the input video signal Yi is not less than the tilt value TILT, the comparator 130 outputs a “low” level signal.
The multiplexer 150 receives the black-stretched signal from the black stretch calculator 140 and the input video signal Yi. If the signal applied from the comparator 130 is a “high” level signal, the multiplexer 150 outputs the black-stretched signal. If the signal applied from the comparator 130 is a “low” level signal, the multiplexer 150 outputs the input video signal Yi.
Accordingly, if the input video signal Yi is less than the tilt value TILT, the black level of the input video signal Yi is stretched. If the input video signal Yi is not less than the tilt value TILT, the input video signal Yi is output as it is without performing black stretch. Therefore, contrast can be enhanced due to black stretch on a bright screen where the mean of the input video signal Yi is greater than the tilt value TILT.
However, according to such conventional black stretch control apparatus, a dark screen where the mean of the input video signal Yi is less than the tilt value TILT gets darker due to black stretch.